1. Field of the Invention
The present invention refers to a method for manufacturing an air filter, to the filter thus obtained and to the apparatus to carry out such method.
2. Background Information
An air filter, particularly for vehicle engines, is usually composed of a filtering element made of pleated paper, shaped as a parallelepiped or a cylinder, to which a perimetral edge is adhered, made of foamed resin, usually polyurethane (PU) or polyvinylchloride (PVC). The foamed resin is generally in the form of flexible foam that fixes shape and dimensions of the filtering element, and simultaneously works as seal gasket when the filter is assembled in its vessel, on board of the vehicle.
The foamed resin edge is usually obtained by placing the paper, adequately pleated, into a die, and then injecting or casting the resin into a die recess running all around the paper. The resin at plastic state sticks to the paper generating an edge that copies the die recess shape.
The employment of this technique, however, provides different kinds of problems. One of these problems is created by the possible nonuniform distribution of the resin along the die recess, particularly in the part that is far away from the injection point. Another problem can be the excessive penetration of the resin into the filtering paper, due to the low resistance opposed by it to the melted resin flow that enters the above-said recess under pressure, during injection.
In the German Patent nr. 2.162.355, a die equipped with side plates is proposed to be used, where such plates limit the contact area between paper element and injected resin, to avoid that the latter migrates on the paper and impregnates its pores. However, though the contact area is reduced, a direct interface is always present between filtering paper and injected resin, so that the chance of migration of the resin through the paper cannot be excluded.